You are right stvmor, without even a psu fan proving some airlow, the p3 will be marginal. The only way to tell how well natural convection will help in the case is to try it. I am currently running a p3 fanless on a desk (no case) and temps are comfortable though.
If it is marginal a single 120mm nexus at 7 or 5v will prove more than enough airflow for all the components - and you won't get a quieter fan than that. It will still give 007 the quietest pc possible.
As for swapping psu fans - i agree it can be a good way of making things quieter without spending money - however it's really important to make sure people know that PSUs are dangerous. The capactiors hold enough charge to kill or at least give a very nasty shock. Often the heatsinks etc are live. Simply turning off the power is not enough - the capacitors will not discharge. Anyone who opens up their PSU needs to take real care.
Also - reducing the airflow to the PSU is venturing into unnown territory. You can't know how well the PSU has been designed. Although a slower fan may cool the main heatsinks sufficiently - some smaller components may no longer have sufficient airflow and could fail. Worst case scenario the PSU failing could take out many of your pc components with it :/
It's worth being aware too that on a 430W seasonic (one of the most efficient PSus out there) Although at higher loads it peaks at over 80% efficient, at the low low loads that 007s pc will use, the psu could be as low as 60% efficient!
the picopsu etc may be more expensive but it certainly gives some bragging rights and makes an interesting project
That's half the fun of modding/overclocking pcs. I'm sure that's why we're all here and not buying Dells
However, a quiet PSU fan moves next to no air (that's why it's quiet!) so your passivly cooled CPU, chipset, memory, hdd etc will not receive ANY cooling because there is NEXT TO NO air flow through the case! 
So you may then need to add cooling fans thus raising the noise levels again 
